Initially, remote sensing of the chemical composition of the Earth atmosphere was focused on the stratosphere: first with the Solar Backscatter Ultraviolet Radiometer- 1 and -2 (SBUV-1 and -2) in the 1970s on-board the American Nimbus 4 and then the series or Total Ozone Mapping Spectrometer (TOMS) instruments to monitor ozone. Stratospheric NO2 – Nitrogen dioxide was measured by a number of NASA satellite instruments since the 1980s, such as the spectrometer on-board Solar Mesosphere Explorer (SME -1981-1989) in limb viewing (Mount et al., 1984), the series of Stratospheric Aerosol and Gas Experiment (SAGE-II/III – 1984-2005) using the Solar occultation technique (Chu and McCormick, 1979, 1986), the Halogen Occultation Experiment (HALOE – 1991-2005) on Upper Atmosphere Research Satellite (UARS) (Gordley et al., 1996), and the Polar Ozone and Aerosol Measurement (POAM – 1993-1995) on-board SPOT- 2 (Randall et al., 1998).

Over the last 22 years, Europe has led efforts to study distributions and amounts of trace constituents (especially NO2) in the troposphere from space. This has started with the launch of the Global Ozone Monitoring Experiment (GOME) on-board ERS-2 in 1995 (Burrows et al., 1999), and then SCIAMACHY on-board ENVISAT in 2002 (Bovensmann et al., 1999). ERS-2 and ENVISAT were lead by the European Space Agency (ESA). Note that SCIAMACHY could also observe many constituents in the stratosphere thanks to its limb and occultation capabilities. GOME and SCIAMACHY relied on spectrometers with a wide spectral coverage to observe multiple atmospheric constituents simultaneously. Both instruments used the nadir technique by measuring backscattered Sun light to determine the abundances of, notably, O3 – Ozone, NO2 – Nitrogen dioxide, and other species. GOME was fully functional until 2003 after which only limited processing was possible until 2011. SCIAMACHY stopped with the end of the ENVISAT mission in April 2012. The follow-up of GOME, GOME-2 selected by ESA and the European Organisation for Exploitation of Meteorological Satellites (EUMETSAT), was launched on-board the operational MetOp platforms (MetOp-A in 2006, MetOp-B in 2012). These platforms also include a thermal infrared emission sensor, the InfraRed Atmospheric Sounding Interferometer (IASI), which has shown high potential for tropospheric chemistry (Clerbaux et al., 2009; Clarisse et al., 2011; Bauduin et al., 2016).

Proposed by the Netherlands and led by the Royal Netherlands Meteorological Institute (KNMI), the Dutch-Finnish Ozone Monitoring (OMI) mission was launched on the NASA Aura payload in 2004 (Levelt et al., 2006). OMI also measures the UV-visible backscattered sunlight in nadir but with a much higher spatial resolution than the previous missions: 13 x 24 km2 (instead of 320 x 40 km2 for GOME and 30 x 60 km2 for SCIAMACHY). OMI has delivered a large amount of data on our tropo- spheric composition during the last 13 years at a global and regional scale with an almost daily-global coverage (Levelt et al., 2017). A continuous NO2 data processing system, named DOMINO for OMI, has been operated by KNMI (Boersma et al., 2007, 2011). These data are freely distributed via the Tropospheric Emission Monitoring Internet Services (TEMIS) website (http://www.temis.nl). DOMINO has recently evolved through the Quality Assurance for Essential Climate Variables (QA4ECV) project (www.qa4ecv.eu)(Lorente et al., 2017).

Overall, these satellite missions are mostly designed for trace gas observations. They are not optimized for aerosol particles, although limited information is available. However, the Aura satellite flies in the coordinated NASA A-Train constellation, where leading aerosol space-borne sensors are also present (e.g. MODIS and CALIOP). The advantage of such a constellation is the synergy between spatially and temporally co-registered measurements from different instruments for aerosol and trace gas retrievals compared to one single satellite measurement.

Most of the dedicated satellite missions have a lifetime of at least 5 years and can provide global coverage with a high frequency and spatial resolution (up to daily-global) (Bovensmann et al., 1999; Levelt et al., 2006; Veefkind et al., 2012). In particular, operational daily maps of NO2 columns by OMI (see http://www.temis.nl) show extensive transport features that are changing from day to day within countries, but also air pollution being transported across national borders (Boersma et al., 2007, 2011).

Tropospheric composition monitoring from space will continue beyond SCIAMACHY, OMI and GOME-2. Under the leadership of the European Commission, the Copernicus programme supports the development of Earth Observation satellite and in situ data (see http://www.copernicus.eu). As part of this programme, the Sentinel satellites are dedicated to provide operational observations of the Earth. Launched on Friday 13th October 2017, the Sentinel-5 Precursor (S5P) mission is the first sentinel focused on the atmospheric composition.

It is a single pay- load including the TROPospheric Ozone Monitoring Instrument (TROPOMI), successor of OMI, and led by KNMI, as Principal Investigator (PI) and the Space Research of the Netherlands (SRON) as co-PI (Veefkind et al., 2012). TROPOMI differs from OMI in a number of important ways: 1) a higher spatial resolution (7 x 3.5 km2 in the UV-vis-NIR, and 7 x 7 km2 in the SWIR) which makes it possible to identify different sources of air pollution such as in a mega-cities , and 2) the observation of CO – Carbon monoxide and the greenhouse gas CH4 – Methane. After 2020, S5P will be followed by Sentinel-4 (S4) (geostationary) and Sentinel-5 (S5) (polar orbiting) (Ingmann et al., 2012).

Meanwhile, international efforts have continued. The NASA/NOAA Suomi Na- tional Polar-orbiting Partnership (SNPP) was launched on the 28th October, 2011, with the Ozone Mapping and Profiler Suite (OMPS) on-board, which includes both a nadir and a limb sensor. It gives then the total atmospheric ozone column as well as its vertical distribution from 15 km to 60 km on a daily basis (https: //jointmission.gsfc.nasa.gov/omps.html). In the next years, NASA will launch the Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument to monitor air quality between North America, Mexico city, Canadian soils and the Atlantic and Pacific ocean from a geostationary orbit (https://fpd.larc.nasa. gov/tempo.html). Similarly, the Geostationary Environment Monitoring Spec- trometer (GEMS) will observe tropospheric composition over South-Korea and the Asia-Pacific region thanks to a UV-visible spectrometer (http://www.ball.com/aerospace/Aerospace/media/Aerospace/Downloads/GEMS_0916.pdf?ext= .pdf).

In the future, we will have an international atmospheric composition observation system from space that will, for the first time combine polar (S5P, S5) and geosta- tionary (S4, TEMPO, GEMS) orbits.